1. Field of the Invention
The present invention relates to a particle monitoring instrument which measures particles peeling off from a processing reactor and particles which are appeared and grow and fall down during processing steps, by means of a light scattering method in-situ and in real time.
2. Description of the Related Art
Particles appeared in a processing equipment during manufacturing steps of a large scale integrated circuit (hereinafter referred to as an LSI) are a major cause of reductions in a yield rate of the LSI and the amount of operational time of the processing equipment. In order to prevent the yield and the amount of operational time from being reduced, instruments to monitor the appearance of the particles have been developed.
These instruments are composed of a laser light source and a photodetector and there are two types of instruments, one of which is installed on an exhaust pipe of the processing equipment, and the other absorbs gas from the processing equipment. Both of these pass a sampled fluid through a space where a laser beam exists, and measure an intensity of scattered light whenever particles pass, and the number of the particles appeared. As conventional examples of these instruments, methods disclosed in Japanese Patent Applications Laid Open No. 4-297852/1992, 3-116944/1991, 63-11838/1988, 5-206235/1993, 5-206236/1993, 7-12707/1995, and 5-288669/1993 and Japanese Utility Model Publication No. 62-37160/1987 can be mentioned.
The methods to measure the particles in the sampled fluid involve a problem that the obvious relations between the number of the detected particles and the yield ratio of the LSI, and the relations between the number of detected particles and the amount of operational time of the processing equipment, were not observed. In order to solve this problem, a measurement of the floating particles appeared in the reactor therein in-situ and in real time has been attempted.
Measurement in this attempt is conducted in the following manner. Windows for introducing a laser light into the reactor and for measuring scattered light are fitted on the processing reactor, the laser light scattered by the particles is recorded on a video tape using a CCD camera, the video tape is reproduced to investigate the time for appearance the scattered light and the intensity change thereof, and finally the appearance of the particles can be estimated.
As examples of these conventional methods, there have been papers, here respectively cited, by Gary S. Selwyn, in Journal of Vacuum Science and Technology, Vol. B9, 1991. pp. 3487-3492 and in Vol. A14, 1996. pp. 649-654. Moreover, there have been papers, here respectively cited, by Watanabe et al., in Applied Physics Letters, Vol. 61, 1992. pp. 1510-1512 and by Shiratani et al., in Journal of Vacuum Science and Technology, Vol. A14, 1996. pp. 603-607.
The foregoing conventional technologies involve the following problems.
In the technologies disclosed in Japanese Patent Applications Laid Open No. 4-297852/1992, 3-116944/1991, 63-11838/1988, 5-206235/1993, 5-206236/1993, 7-12707/1995, and 5-288669/1993 and Japanese Utility Model Publication No. 62-37160/1987, which are mentioned as the conventional examples, the methods to measure the particles in the sampled fluid are described. Since these methods adopt measuring the scattered light in sampled fluid, it is difficult to specify the origin of the particles. Therefore, there is a problem that it is difficult to obtain a relation between the amount of operational time of the equipment and the amount of appeared particle and a relation between a yield ratio of the LSI and the amount of appeared particle.
In the papers that are respectively cited, by Gary S. Selwyn, in Journal of Vacuum Science and Technology, Vol. B9, 1991. pp. 3487-3492 and in Vol. A14, 1996. pp. 649-654, by Watanabe et al., in Applied Physics Letters, Vol. 61, 1992. pp. 1510-1512 and by Shiratani et al., in Journal of Vacuum Science and Technology, Vol. A14, 1996. pp. 603-607, the particles floating in the processing reactor are detected by the laser light scattering method, and measured spatial distribution of the particles and its change with time. However, it is difficult to know whether the observed particles cause the faults on wafers, to know the origin of the particles and to know which paths the particles travel to reach a wafer. For these reasons, the origin of the particles causing defective patterns on a wafer product can not be specified. Therefore portions and parts of the processing equipment to require the reform can not be recognized. And the measurement of particle appearance has been cleaning of the equipment. Specifically, there has been no policy to inhibit the appearance of the particles, so that the amount of operational time of the processing equipment cannot be improved owing to cleaning of the reactor and preparative operation.
When the measurement of the spatial distribution of the particles, the way of detecting particles adopts either the laser light scanned spatially or the laser light expanded spatially. In this case, distances from places where laser light scattering occur to the detector are different, and intensities of the scattered lights at the detector is inversely proportional to a square of the distance from the scattering point to the detector. Specifically, to estimate the particle size from the intensity of the scattered light, it is necessary to correct the intensity of the scattered light depending on the distance. However, such correction has not heretofore been performed.
Moreover, to estimate the particle size form the intensity of the scattered light, it is assumed that the shape of the particle is perfectly spherical. However, it has been known that many of the particles appeared on LSI manufacturing steps are in a flasky form and in a needlelike form. For the particles of such shapes, the intensity of the scattered light greatly depends on an arrangement of a direction of incident beam and particle. Therefore, by the estimation of the particle size assuming a perfect sphere particle, errors of a particle size, a distribution of particle size, and a numerical density of the particles become large.
Currently spatial distribution of the particles using the light scattering method is measured as follows. A specifically polarized light is introduced into the processing reactor and the changes of the polarization of the light scattered by the floating particles are measured. Only one wavelength of the light from the light source used is used. When a size of the observed particle is smaller than the wavelength of the irradiation light, the intensity of the scattered light is estimated with a Rayleigh scattering formula. When the size of the particle is larger than the wavelength of the irradiation light, a Mie scattering formula is employed. Although the Mie scattering formula gives a strict solution, its equation is complicated so that a long time is required for a numerical calculation. Therefore no positive information on the size and numerical density of particles, is available on real time, from the results of the scattered light intensity measurements.